Plain English Meaning
A device that keeps every engine spinning at exactly the same speed and also lines up the blades of each propeller in a set pattern, so the airplane runs more smoothly and quietly.
Full Definition
A system installed on multi-engine airplanes that not only matches the rotational speed (RPM) of all propellers but also controls the angular position, or phase relationship, of each propeller's blades relative to the others to minimize cabin noise and vibration.
Context Anchor
You encounter this in multiengine propeller airplane operation, especially when setting propeller controls for cruise and reducing the uneven sound or vibration between engines.